The present invention relates to a steering system for a tracklaying vehicle, which includes a pair of brakes provided for right and left output shafts and is designed to transmit the driving force of a hydraulic motor to the respective output shafts through a differential steering means.
In a known steering system for a tracklaying vehicle such as a bulldozer, the hydraulic motor is driven by a hydraulic pump actuated by the engine and the output of the hydraulic motor is transmitted to the right and left output shafts through a differential steering means such as a planetary gear mechanism, thereby making the rotational speeds of the right and left sprockets different from each other to turn the vehicle.
Such a steering system for a tracklaying vehicle suffers from the problem that since the hydraulic motor receives a reaction force caused by the turning resistance of the vehicle when the vehicle turns and the reaction force acting on the hydraulic motor increases as the radius of turn of the vehicle becomes small, a radius of turn exceeding the capacity of the hydraulic pump and the hydraulic motor cannot be obtained. In addition, the steering system employing the hydraulic motor also presents such a problem that all the power required for turns is obtained from oil pressure and therefore allowance has to be made for engine power.
An attempt to solve the above problems has been made in Japanese Patent Application No. 63-235173 (1988). This publication teaches a system in which the closed circuit for the hydraulic pump and the hydraulic motor is provided with right and left steering brake valves and a transverse shaft is provided with a pair of brakes. In this system, when the operating position of the steering lever becomes more than or equal to an intermediate operational amount, the output of the hydraulic motor is set to zero and either the right or left sprocket is braked, whereby the reaction force is prevented from working on the hydraulic motor when a turn with a small turning radius (e.g., pivot turn) is carried out.
The technique disclosed in the above publication, however, has revealed the following problem. Regarding the relationship between a timing for braking a sprocket by a brake and a timing for disconnecting the hydraulic motor from a driving power source, if the braking timing is slower than the disconnecting timing, the relative speed of the right and left sprockets is dependent only on the load of the road surface applied to the respective crawler belts, so that the radius of turn of the vehicle cannot be made to be constant. On the other hand, if the braking timing is quicker than the disconnecting timing, the vehicle gets a great shock at the instant the vehicle starts a pivot turn, in some cases depending on the relationship between the timings.
To solve such a problem, the present inventors have already suggested a steering method for a tracklaying vehicle in the previous invention (Japanese Patent Application No. 2000-83329). According to this method, when the vehicle makes a pivot turn, after actuation of the brake for either one of the right and left output shafts and a transmission of a driving force from the hydraulic motor to the differential steering means have been carried out concurrently for a specified period of time, the transmission of the driving force from the hydraulic motor to the differential steering means is interrupted. The previous invention also discloses a method for controlling the rotational speed of the engine so as to be less than or equal to a specified value during a pivot turn of the vehicle in order to prevent an unfavorable situation such as when either one of the drive shafts is stopped by actuation of its corresponding brake, the hydraulic motor receives a driving force from the other drive shaft being rotated and, in consequence, the rotation of the motor overruns.
However, in cases where the motor overrun prevention method disclosed in the previous invention is adopted, there arise other problems. Specifically, the rotational speed of the motor deviates from the desired value in some cases depending on the surface condition of the road on which the tracklaying vehicle is traveling, or the engine may be excessively throttled down, resulting in a shortage of power during a pivot turn.
The invention is directed to overcoming the foregoing problems and a primary object of the invention is therefore to provide a steering system for a tracklaying vehicle which is capable of preventing overrun of the motor during a pivot turn irrespective of the load of the road surface and does not cause a shortage of power during a turn.
The above object can be accomplished by a steering system for a tracklaying vehicle according to the invention wherein a pair of brakes are provided for right and left output shafts and the driving force of a hydraulic motor is transmitted to the right and left output shafts through differential steering means, the system comprising:
(a) brake actuating means for independently actuating the right and left brakes;
(b) driving force transmission interrupting means for interrupting a transmission of a driving force from the hydraulic motor to the differential steering means;
(c) pivot turn control instruction signal detecting means for detecting an issue of a vehicle pivot turn control instruction signal;
(d) vehicle speed detecting means for detecting the vehicle body speed of the vehicle;
(e) controlling means for controlling the brake actuating means and the driving force transmission interrupting means in response to an output from the pivot turn control instruction signal detecting means such that after actuation of a brake for either one of the right and left output shafts by the brake actuating means and a transmission of a driving force from the hydraulic motor to the differential steering means are carried out concurrently for a specified period of time, the driving force transmission interrupting means interrupts the transmission of the driving force from the hydraulic motor to the differential steering means,
wherein the controlling means controls an engine such that the vehicle body speed of the vehicle detected by the vehicle speed detecting means during a pivot turn of the vehicle becomes equal to a specified value.
In the invention, during a pivot turn, actuation of a brake for either one of the right and left output shafts and a transmission of a driving force from the hydraulic motor to the differential steering means are concurrently carried out for a specified time and thereafter, the transmission of the driving force from the hydraulic motor to the differential steering means is interrupted thereby bringing the hydraulic motor into its free condition. Accordingly, trouble caused by an unfavorable situation such as when the timing for the brake actuation is delayed and the free condition of the hydraulic motor continues until the brake is actuated can be avoided. In other words, the problem of having the relative speed of the right and left sprockets be dependent upon only the load of the road surface exerted on the respective crawler belts can be avoided. In addition, that the present invention prevents the vehicle from getting a great shock at the instant of a pivot turn. Further, during a pivot turn, the transmission of a driving force from the hydraulic motor to the differential steering means is interrupted so that a reaction force from the road surface does not act on the hydraulic motor, and therefore, it is possible to make a pivot turn with a conventional hydraulic motor of a small capacity. Since the controlling means of the invention controls the engine such that the vehicle body speed of the vehicle detected by the vehicle speed detecting means during a pivot turn of the vehicle becomes equal to a specified value, the present invention also avoids the possibility that while one of the drive shafts is stopped by actuation of its corresponding brake, the hydraulic motor receives a driving force from the rotating one of the drive shafts, resulting in overrun of the rotation of the motor. Additionally, since the engine is controlled based on a detection signal indicating the vehicle body speed of the vehicle, the engine control can be carried out irrespective of the load of the road surface in any conditions so that a shortage of power can be reliably prevented during a turn.
Preferably, in the invention, the vehicle body speed is detected by detecting the rotational speed of the output side of the transmission, and according to the detected vehicle body speed, the controlling means controls an engine rotation instruction value. It should be noted that the vehicle body speed may be detected by the output shaft of the transmission, a bevel shaft, or a transverse shaft as far as these shafts are on the output side of the transmission.
In this case, the controlling means preferably executes the engine control based on a map showing the relationship between vehicle body speed and engine rotation instruction values. The map is comprised of a first map for determining an engine rotation instruction value corresponding to a detected vehicle body speed and a second map for compensating for the deviation of the engine rotation instruction value determined by the first map from a target vehicle body speed. With this arrangement, after a value approximate to an engine rotation instruction value corresponding to a target vehicle body speed has been obtained by use of the first map, the deviation of the current vehicle body speed from the target vehicle body speed can be compensated for, so that not only the hunching phenomenon but also excessive throttling of the rotational speed of the engine can be prevented, thereby converging the vehicle body speed on the desired value within a short time.
Preferably, the system of the invention includes tractive force detecting means for detecting the tractive force of the vehicle such that when the tractive force detected by the tractive force detecting means becomes more than or equal to a specified value during a pivot turn of the vehicle, the controlling means shifts the transmission to a lower speed range. With this arrangement, where a great tractive force is needed because of turning resistance when the transmission is placed in a high speed range, higher turning efficiency can be achieved by obtaining a greater tractive force through shifting of the transmission into a lower speed range.
In this case, when shifting the transmission to a lower speed range, it is preferable to carry out gear shift with one selected from a plurality of speed range characteristic lines each representing a tractive force characteristic versus vehicle body speed, the selected speed range characteristic line providing a greater tractive force value for the same vehicle body speed than others. This enables turning operation with the highest efficiency.